1. Field of the Invention
The present invention relates to composite fiber and resin reinforcements for strength members. More particularly, the present invention relates to such composite reinforcements having improved strength over prior composite reinforcements. Even more particularly, the present invention relates to composite fiber and resin reinforcements to use in reinforced wood and wood based products such as reinforced structural laminated timber, reinforced structural lumber and reinforced glulam.
2. Description of the Prior Art
The concept of reinforcing products with fibers to strengthen the products in order to become structural members is known in the art. The advantage of doing so, the method for attachment, and conventional methods for making the structural members are also established in the art. For example, U.S. Pat. No. 5,928,735 (Padmanabhan, et al.), U.S. Pat. No. 6,179,942 (Padmanabhan, et al.), U.S. Pat. No. 5,456,781 (Tingley) and U.S. Pat. No. 6,105,321 (KarisAllen) may be considered relevant in the art. It is known that the use of composites formed by the pultrusion process is a convenient way to attain the use of fibers for reinforcing products. This is further disclosed in U.S. Pat. No. 6,037,049 (Tingley) and U.S. Pat. No. 5,362,545 (Tingley).
Pultrusion is generally defined as a continuous process of manufacturing of composite materials with constant cross-section whereby reinforced fibers are pulled through a resin, possibly followed by a separate preforming system, and into a heated die, where the resin undergoes polymerization and where the reinforced plastic is shaped and the resin is cured. Pultrusion is known for the ability to fabricate a continuous length of reinforced plastic and to accommodate desired placement and orientation of fibers, which allows for the mechanical properties of the pultruded part to be designed for a specific purpose or application. Pultruded parts comprise longitudinally aligned fibers for axial strength and obliquely aligned fibers for transverse strength. Many resin types may be used in pultrusion, including polyester, polyurethane, vinylester and epoxy.
Reinforcements for structural members have been manufactured using pultrusion processes. This process generally involves wetting fibers with resin and pulling the wet fibers through a mold where the resin is cured by heating the resin. Exemplary pultrusion processes are disclosed, for example, in U.S. Pat. No. 2,419,328 (Watson, et al.), U.S. Pat. No. 2,684,318 (Meek), U.S. Pat. No. 3,895,896 (White, et al.), U.S. Pat. No. 5,286,320 (McGrath, et al.), U.S. Pat. No. 5,374,385 (Binse, et al.), U.S. Pat. No. 5,424,388 (Chen, et al.), U.S. Pat. No. 5,556,496 (Sumerak), U.S. Pat. No. 5,741,384 (Pfeiffer, et al.) and U.S. Pat. No. 5,783,013 (Beckman, et al.).
Another type of pultrusion process, often referred to as continuous lamination, involves spreading resin on a film, such as MYLAR®, adding fiber materials to the spread resin and then adding a top cover film to form an envelope that essentially becomes a flexible mold. This “sandwich” configuration is shaped by tension and mechanical forces, and is then pulled through an oven to cure the “sandwich” configuration into a desirable form.
A third variation of pultrusion provides placing the fibers under tension, saturating the fibers with photo-initiated resin, pulling the fibers through a series of sized dies or nip rolls to form the fibers into a bundle or web, and then exposing the fibers to high intensity ultraviolet light to initiate curing. A surface coating is then applied and cured to provide a desired resin rich surface. This process has been used in forming artificial leather and strengthening members of fiber optic cables. Exemplary variations of this process are disclosed in U.S. Pat. No. 244,872 (Fischer), U.S. Pat. No. 4,861,621 (Kanzaki), U.S. Pat. No. 5,700,417 (Femyhough) and U.S. Pat. No. 6,893,524 (Green).
A fourth variation of pultrusion provides placing the fibers under tension, saturating the fibers with thermo-reactive resin, and pulling the fibers through a series of sized dies to form the fibers into a round bundle while they are exposed to elevated temperatures, such as those found in an oven. This process has been used for making fishing rods, and has also been adapted for manufacturing fiberoptic cable strength members.
Thermoset polyurethane resin has shown the ability to be formulated to adjust the flexibility, elasticity and tensile properties. This elasticity has provided a more secure bonding to man-made fibers, such as aramid and nylon, as shown in U.S. Pat. No. 4,695,509 (Cordova, et al.). The adjustable formulations are highlighted in several patents that show the capabilities for making a very tough product, as exemplified in U.S. Pat. No. 6,787,626 (Dewanjee).
Some difficulties with the current art have been identified. For example, the pultrusion process as discussed above is limited. The closed die method becomes less efficient when the thickness of the fibers is at 0.030″ and less because of the lack of space for foreign objects, crossed fibers, fiber knots and splices. This is complicated with the use of rigid resins like polyester, vinyl ester, epoxy, acrylic and others as the interlaminate shear is reduced and the product can split longitudinally (i.e., parallel to the fibers) quite easily while in process, as well as during post processing. In this instance, the processing speed will also be kept to less than 10 ft. per minute.
A method for providing a greater interlaminate shear (to reduce the splitting) in thicknesses below 0.030″ comprises adding a web of materials to the structure. The web of materials has fibers in directions other than parallel to the longitudinal fibers. In doing this, an amount of the longitudinal fibers must be replaced in a normally disproportional amount, thus reducing the product strength and requiring a thicker laminate to enable the same reinforcing function.
Many of the open (i.e., without a die or cover envelope) processes have been substantially limited to the making of round products, and more specifically to the making of products having a thickness below 0.035″ and when substantially all of the fibers are longitudinal fibers. In addition, the more commonly used resins of polyester and vinyl ester release harmful emissions that increase when cured in an open process and are required by state and federal laws to be limited.
Existing patents and procedures for reinforcements bonded to wood do not include the use of thermoset polyurethane resin. Thermoset polyurethane resin has a number of desirable characteristics including good flexibility, elongation and resistance to corrosion. However, the use of thermoset polyurethane resin for reinforcements has been thought to be too flexible, too hard to use as a matrix in the reinforcement, and to display poor bonding to adhesives. It should be noted that there is a difference between thermoplastics and thermosets. Thermoplastics usually contain additives to change the properties of the material such as polypropylene while thermosets usually contain catalysts that change the state of the material at the molecular level. Thermoplastics can be re-melted and recycled fairly easily. Thermosets typically are cured and molded into shape and are not recycled as easily. U.S. Pat. No. 6,749,921 (Edwards et al.) specifically states that the use of a thermoplastic polyurethane composite provides an avenue for the shaping of and hammering nails into the wood composite, which are not possible using fiber-reinforced thermoset composites due to their brittleness. Therefore, Edwards et al. teaches away from the use of a thermoset polyurethane composite in the wood composite industry.
There is thus a need for an improved composite fiber and resin reinforcement for strength members for use in reinforced wood and wood based products having good flexibility, elongation and resistance to corrosion.